Numerous door intercoms for dwellings of that type are known in which sound and data are conveyed between the street panel and the indoor set by one or more electric cables.
The electrical power consumed by the electric circuit of the latch is relatively large and it is typically greater than about 10 watts.
Manufacturers of electric latches therefore propose powering them from a transformer whose primary is connected to the mains supply, and whose secondary is capable of delivering about 20 watts of power.
Laying electric cables between the street panel and the indoor set and for powering the electric latch constitutes a problem when no provision was made therefor in the initial construction of the dwelling.
A similar problem is encountered when a cable needs to be laid between a call button and a doorbell located inside the dwelling. Devices, sometimes known as "wireless chimes", have been available for a long time, in which the traditional wire link between the call button and the doorbell is replaced by a radio link.
However, in a wireless chime the information transmitted by radio is one-way and very simple since all that is required is to trigger the doorbell under remote control, whereas with a door intercom information must travel in both directions, and the information concerned is more complex since it must not only enable dialog to be established between the visitor and the occupier, but it must also enable the latch to be remotely controlled when the occupier seeks to let the visitor in.
So far as the Applicant is aware, no wireless intercom has yet been made available.
Known techniques such as those implemented in wireless chimes provide no solution to the problem of electricity consumption by the street panel, and would require the use of batteries of large capacity and of high cost, or else battery replacement that is so frequent, or a number of batteries that is so large, as to remove any practical advantage in having a wireless link.
In addition, apart from the problem of street panel battery life, known wireless telephony techniques do not provide a solution to the problem of transmitting information such as a latch-opening command, since such transmission must be insensitive to interference in order to avoid any untimely opening of the latch which could jeopardize the security of the dwelling.
Finally, the wireless link between the street panel and the indoor set must be a long-range link in that it must work through the walls of the dwelling.
Unfortunately, the known solution of increasing the range of a transmitter by increasing its power is incompatible both with the requirement for battery lifetime in the street panel and with complying with transmission standards laid down by the authorities which put a severe limit on maximum authorized power.
French patent application No. FR-A-2 593 339 and German patent DE-C-44 08972 describe door intercoms in accordance with the precharacterizing portion of claim 1, and capable of controlling a latch by radio waves.
Nevertheless, in those known devices, the latch is not powered by an stand-alone power supply contained in the street panel and it is necessary to use a power supply cord to connect the latch or the street panel to the electricity mains supply, with all the drawbacks associated with installing such a cord unless installation thereof was planned at the time the dwelling was built.